This project involves the study of a series of porphyrins which, in the presence of light, catalyze production of singlet oxygen. The latter causes damage to many biologic structures. In the present work, we have identified alterations in membrane structure and function in cells exposed to porphyrins in the presence of light. Further studies are designed to characterize sites of porphyrin binding, to identify porphyrin structures with optimal toxicity, to identify modes of porphyrin accumulation, and to gain an understanding of the types of porphyrin-induced cell damage associated with the anti-tumor action of these agents. The ultimate goal of this work is the selection of porphyrins optimally useful in treatment of neoplastic disease in man.